1. Field of the Invention
The present invention relates to an image forming system employing an effective optical scan-line control device, in particular, to an optical scanning control method, optical scanning device, or an image formation device, which employs an effective optical scan-line control device.
2. Description of the Related Art
In an image formation device, such as a laser printer, an optical plotter, a digital copier, or the like, an optical scanning device is employed. In the optical scanning device, a beam emitted from a light source is deflected by an optical deflection scanning device such as a rotation multiple mirror such as a polygon mirror or the like, is focused by a scanning image-formation optical system, such as an fθ lens into an optical spot, onto a scanning surface.
In the image formation device employing the optical scanning device, an image formation process is performed in which optical scanning is performed with the optical scanning device. There, the quality of the image formed depends on the quality in performance of the optical scanning. It depends on scanning characteristics on a main scanning direction and scanning characteristics on a subscanning direction of the optical scanning device. As one of the scanning characteristics on the main scanning direction, a uniform velocity performance is known.
In order to achieve the satisfactory uniform velocity performance in the optical scanning operation, the deflection of beam is performed in a uniform angular velocity in case of employing a rotation multiple mirrors for example. There, a scanning image-formation optical system having fθ characteristics is used. However, the perfect fθ characteristics may not be achieved there, and also, some other performances are also required from the scanning image-formation optical system.
A scan line bending phenomenon is known as one problem occurring in regard to the scanning characteristics on the subscanning direction. A scan line is drawn by an optical spot on the scanning surface, which should be a straight line ideally. However, due to a manufacture working error, an assembly error, etc., usually the scan line inevitably bends on the scanning surface. As one form of such a scan line bending phenomenon, inclination of scan line is known in which the scan line does not cross at a right angle with respect to the subscanning direction.
In case of providing an angle in the subscanning direction between a direction of a beam incidence onto an image-formation mirror and a direction of the same reflected thereby in the scanning image-formation optical system, the scan line bending phenomenon occurs inherently. In case the scanning image-formation optical system is formed by a lens system instead, occurrence of scan line bending is unavoidable in a multi-beam scanning form which carries out optical scanning with a plurality of optical spots separate along the subscanning direction.
Distortion along the main scanning direction arises in a resulting image formed when the above-mentioned uniform velocity performance of optical scanning is not perfect. When scan line bending occurs on the other hand, a distortion along the subscanning direction arises in the resulting image formed.
In case a monochrome image is formed by a single optical scanning device, imperfection in the uniform velocity performance may not cause a serious distortion in the resulting image in terms of visual performance of human eyes as long as the imperfection and scan line bending phenomenon are controlled to a certain degree. However, in case a color image is formed by a so-called tandem-type image formation device in which images of primary color components first formed are combined so as to provide a full-color image, serious problems may likely to occur as will now be described.
To form separate color component images of respective three colors of magenta, cyan and yellow or four colors which also includes black, and, after that, to produce a full-color image in combination thereof by piling up these color component images in a color copying machine, etc. is known. One example of a machine which performs such a color image forming process is the tandem-type image formation device mentioned above in which color component image of each color is formed onto a separate photoconductor with a separate optical scanning device. In such a configuration, some abnormality may occur in a resulting image, when a color deviation due to difference in a manner of scan line bending occurring on each color component image between the respective color components. Thereby, image quality in the finally obtained color image is degraded. The term of color deviation includes a phenomenon in which colors occurring in the finally obtained color image are not those which are desired originally.
Recently, as one trend in manufacture of the optical scanning device (a lens or so), such a special surface as an aspherical surface is employed as a surface of an optical system used there. In this regard, an image-forming optical system made of a resin or plastic material takes an attention as a method of enabling easy manufacture of such special surfaces at low costs and thus advantageous in a recent mass production environment.
As for the image-forming optical system of resin or plastic material, the optical characteristics tend to change in response to change in ambient temperature or humidity, which may result in change in the above-mentioned optical characteristics whereby the uniform velocity performance may be degraded or the scan line bending phenomena may likely to occur. As a result, when performing color image formation of dozens of sheets continuously for example, the temperature inside the machine rises by the continuation operation of image formation processing, and the optical characteristics of the image-forming optical system there may change. Thereby, the uniform velocity performance or scan line bending manner on the optical scanning device for each color component change gradually. As a result, the color tone may completely differ between a resulting color image obtained at the beginning of the above-mentioned continuous image forming process and a resulting color image obtained at the end of the same process.
The above-mentioned tandem-type image formation device will now be described in detail. There, four drums of photoconductors for respective color components are arranged in a recording paper conveyance direction. Each photoconductor drum is exposed by a corresponding optical scanning device, and a latent image is formed on the photoconductor. The thus-obtained latent images are visualized by toners of the respective color components, i.e., yellow, cyan, magenta and black. Then, these visualized images are transferred onto a recording paper one by one in a piling-up manner, and, thus, a full-color image is obtained on the recording paper. Such a configuration of image formation device is put in practical use as a digital color copying machine or a color laser printer.
Such an image formation device of a 4-drum tandem type is advantageous in comparison to another type of color image formation device in which a (electrostatic) latent image for each color component is formed on a single common photoconductor one by one using a single common optical scanning device. In this type of image formation device, the latent image thus formed is visualized one by one as a visible image of yellow, magenta, cyan, and black, and, then, the thus-formed visible image is transferred onto a recording paper, one by one. In comparison with this type of machine, the tandem-type machine is advantageous in that full-color image formation can be archived theoretically at the same rate as that in case of monochrome image formation. Thus, high-speed color image formation or printing is achieved by the tandem-type machine. However, in the tandem-type machine, since a separate scanning image-formation optical system is provided for each of photoconductor drums, the above-mentioned color deviation may likely to occur as mentioned above, when a visible image (toner image) is transferred on the same recording paper from each separate photoconductor drum in the piling-up manner.
As causes of the color deviation along the subscanning direction, the following ones are expected: Rotation speed variation in the drum-type photoconductors; positional deviation among scan lines drawn by the optical scanning devices for respective color components; deviation in manner of scan line bending among the respective color components; shift of scan lines or change in manner of scan line bending due to environmental transition or temperature rise according to progress of the above-mentioned continuous image formation process, and so forth. Especially, the temperature rise according to the progress of image formation process may cause serious optical performance transition in optical devices made of resin/plastic materials.
As a method of reducing the color deviation, various methods have been proposed. In one plan, disclosed by Japanese patent No. 3262409, when the temperature in a machine exceeds a threshold, the amount of toner image transfer registration deviation is detected, and, based thereon, an actuator is driven so as to correct the positional deviation. In another plan disclosed by Japanese laid-open patent application No. 2001-133718, positional adjustment of an optical scanning device provided for each photoconductor drum is performed together with a housing thereof with respect to the photoconductor drum. In another plan, a long lens included in the optical scanning device is deformed so as to correct the scan line bending as disclosed by Japanese laid-open patent application No. 10-268217.
According to the above-mentioned plan of Japanese patent No. 3262409, it may be difficult to carry out a high-speed drive of the actuator which drives a long heavy mirror, and, thus, when a temperature inside the machine changes rapidly at a time of continuation image formation, it may be difficult to achieve a timely response thereto.
In the method of Japanese laid-open patent application No. 2001-133718, the cost may increase as the mechanism for the adjustment tends to become complicated. Moreover, the scan line bending phenomena occurring gradually due to temperature change or the like may not be controlled well.
In the method of Japanese laid-open patent application No. 10-268217, it may be effective to well correct the scan line bending at a time of initial setting state. However, it may be difficult to deal with a problem occurring gradually due to a temperature change, or the like, occurring at a late stage.